GEOL100
3-1-10
Weathering and Soils

We now embark on a study of sedimentary rocks, rocks that are composed of the fragments of preexisting rocks. Before you can make a sedimentary rock, you must do four things to preexisting rocks:

Weather them so that at least parts of them disintegrate

transport the products of weathering

deposit them

cement them together with mineral precipitates.

Today, we will examine the first of these processes: Weathering

Weathering: The process by which rocks and minerals at the Earth's surface are physically and chemically broken down. All weathering involves the rock's reduction into smaller (sometimes molecule-sized) pieces.

Erosion: What weathering is not - physical removal of the weathered material.

Weathering is a combination of both chemical and mechanical processes:

Mechanical weathering
The processes by which rock is mechanically broken into smaller pieces.

Mechanical Weathering:

This occurs through a distinct sequence of events. First, joints (i.e. cracks) form in rocks at or near Earth's surface. Then, those joints are enlarged by a set of secondary processes.

Unloading: Joints originally form when decompression causes rocks to crack. This typically occurs as the forces of erosion remove the rock's overburden (i.e. the stuff on top of it.) "Exploding rocks" in deep mineshafts are also fracturing as a result of uneven unloading. Forms of jointing include

Exfoliation - the spalling off of sheets of rock from the outer surface of parent rock. Structureless rocks like granite and other intrusive rocks tend to weather into characteristic rounded shapes through this process.

Frost wedging - Unlike most substances, water expands when it freezes. Thus, water that invades joints during warm months tends to wedge them apart, enlarging them during winter.

Root wedging - On both a large and small scale, plants and fungi invade joints and the spaces between grains and wedge them apart. In cold climates, they act in concert with frost.

Animal activity - creates and enlarges openings in rock.

In addition to these, there are other processes that contribute to weathering:

Salt wedging - In arid climates, dissolved salt precipitates in the small pores and joints between grains, wedging them apart on a small scale.

Thermal expansion - Natural fires quickly heat up the outside layers of a rock, but this heat may not penetrate far inside. The result is that the outside layers expand more quickly than the inside, causing fracturing. The common experience of a rock "exploding" in a campfire is an example of this.

Talus: The products of these forms of mechanical weathering often pile up in large cones of "talus" at the base of slopes mountainous regions.

Of course, the properties of the rock being weathered will strongly influence the character of the weathering:

Chemical weathering

If all weathering were mechanical, we would expect weathering products to resemble parent materials in miniature. Is this what we see? Consider my Hawaii trip. Lots of quartz sand beaches, little olivine.

What can account for this? Consider the following fact: Many minerals form under (are at home in) conditions very alien to the earth's surface. When exposed to surface conditions , they are rapidly altered chemically. (In contrast, those that are more nearly "at home" on Earth's surface tend to resist weathering longer. Thus, olivine doesn't last long but quartz hangs around.) This leads us to the topic of chemical weathering, the process whereby rock materials are decomposed by chemical alteration of the parent material.
Mechanisms of chemical weathering: Having said this, lets consider some specific ways in which minerals equilibrate with the environment.

Dissolution - chemical reaction between minerals and acidic or alkaline water may radically alter the mineral's solubility.

Note, most rainwater is slightly acidic because dissolved CO2 interacts with water to form carbonic acid.

CO2 + H2O --> H+ + HCO3-

This, in turn, dissolves the calcite in limestone, yielding open cavities that can be the size of large buildings.

In this case, the orthoclase is replaced by kaolinite, and K+ and SiO2 ions in solution. Whereas orthoclase was a blocky, three dimensional mineral, kaolinite is essentially a 2 dimensional mineral. Consequently, its mechanical properties are very different and much less resistant to mechanical weathering.

Oxidation. Although chemists have a technical definition, geologists tend to use it in a vernacular sense to refer to reactions with O2. Most oxidation is carried out by O2 dissolved in water. Ferric iron (Fe2+) is an important reagent.

Fe2+ + O2 + 2H20 ---> 4(Fe3+)O(OH)

In this case, the ferric iron that could be the cation in a mineral like pyrite or biotite is removed and incorporated in goethite, a more soluble form of iron oxide. (It goes without saying that the presence of goethite implies that water was once also present, thus the search for goethite and similar minerals on Mars is a major goal of the Mars Expidition Rovers.

Hydration. Some minerals can entrap water molecules in their lattice. Anhydrite (CaSO4) for instance, can accept two water molecules to become gypsum (CaSO4.2H20). (This is the process by which plaster of Paris accepts water then hardens.) This transformation involves a change in volume that affects a rock's mechanical properties.

Organisms can also change the chemistry of their environment. Plants typically lower the local pH (i.e. make it more acidic.) Bacteria are notorious for acting as catalysts in various geochemical processes.

Soils:

An important consequence of the factors we just listed is that in some regions, weathering products hang around and continue to interact with weathering agents and parent material. The result is soil. Terms:

Definition - That part of the regolith that consists of weathered rock, water, air and HUMUS, organic matter derived by decay of plant material, that can support rooted plants. (They need nutrients moisture and mechanical support

Major attribute
-soil is the link between the realm of the solid Earth and the biosphere.

The ideal soil profile -the succession of distinctive horizons (a new term for layers or strata, when applied to soils) from the surface down to the bedrock (unaltered parent material). All soil profiles contain the following, in some form or other:

A horizon may be thick, but is lacking in soluble materials. What's present is typically rich in insoluble iron and aluminum oxides, including bauxite (aluminum ore).

B horizon is absent

Paleosols: A soil requires the constant input of organic material, so it can be "killed" if it is deprived of this. Climate change and burial by sediments can both kill soils. Buried ancient soils are called paleosols. These can be used to reconstruct ancient environments. Laterite paleosols can be commercial sources of aluminum.